Current state-of-the-art photovoltaic (PV) modules based on silicon (Si) wafer technology employ a front glass/encapsulant/wafer/encapsulant/backsheet structure. Polyvinyl fluoride (e.g., Tedlar® from DuPont) is the most commonly used backsheet in wafer-based Si PV modules. This material is inherently transparent for water vapor and gases—i.e., it does not enable or maintain a hermetic seal. Thin-film PV modules based on amorphous Si (a-Si), CdTe, or copper indium gallium selenide (CuInxGa1-xSe2 or CIGS) most often utilize a glass-glass architecture with one of the glass sheets either serving as the substrate (CIGS) or superstrate (a-Si, CdTe) onto which the active PV material is deposited directly. The function of the second glass sheet is to serve as either the protective top (CIGS) or back cover (a-Si, CdTe). Both sheets of glass are held together with a single encapsulant layer. For practical purposes the 1.8-4 mm-thick sheets of glass are impermeable to water and gases. However, moisture and gas ingress can occur along the edges of the glass/encapsulant/glass sandwich. Exposure to such moisture can result in issues such as corrosion, decreased minority carrier lifetime, and increased series resistance, deleteriously impacting the performance and lifetime of the PV module. To slow down moisture ingress and gas permeation, butyl-based edge seals are typically applied, as butyl rubber has water vapor transmission rates (WVTR) on the order of 10−6 g/m2-day, orders of magnitude below the WVTR for typical encapsulant materials.
While the WVTR value for butyl is considered to be adequate for long-term protection, the overall vulnerability of a device to moisture usually derives less from the bulk diffusivity of the butyl than from the durability of the interface between the butyl and the glass. Adhesion promoters to establish coupling of the polymer to the glass are typically added to the butyl, but the resulting bonds tend to be weak and easily damaged by ultraviolet radiation, prolonged exposure to elevated temperatures, and humidity—i.e., conditions in which PV modules are commonly deployed. Stresses present between the two sheets of glass can further strain the glass/butyl interfaces. Hence, even for such edge-sealed glass-glass structures, moisture penetration is a common field failure mode. Thus, there is a need for hermetically sealed modules and techniques that enable environmental protection of thin-film PV devices and which are superior to current butyl-rubber-based solutions.